1. Field of the Invention
The present invention relates to solenoid proportional pressure control valves and other spool valves in which the pressure in a pilot chamber is varied in accordance with the strength of an electric current applied to the solenoid to thereby control the fluid pressure or flow rate. It particularly relates to a spool valve which prevents internal orifices of the valve from becoming clogged by particles of foreign matter such as dirt and the like and also prevents sticking of the slide caused by particles being caught between the valve housing and the spool.
2. Description of the Prior Art
Spool type control valves such as, for example, solenoid proportional control valves, include the solenoid proportional pressure control valve described in Japanese Unexamined Patent Publication No. 62(1987)-261782. The valve of the disclosure effects pressure control in accordance with the strength of an electrical current flowing in a solenoid. The arrangement of this valve is shown in FIG. 10.
In FIG. 10 a spool B is slidably provided in a spool passage al formed in a valve housing A. A horizontal opening b1 is formed in the right-hand end of the spool B to constitute a pilot chamber C. The pilot chamber C communicates with a pump port a2 via an orifice b2 and a large-diameter opening b3, and also communicates with a return port (not shown) via the pilot valve (not shown) of a solenoid.
In the spool control valve such as, for example, a solenoid proportional control valve thus constituted, when the solenoid is energized a pilot valve reduces the area of communication between the pilot chamber C and the return port, producing a rise in pressure in the pilot chamber C. This pressure causes the spool B to be moved to the left against the force of a spring D and brings the pump port a2 and an actuator port a3 into communication, and hydraulic fluid from the pump port a2 flows via the actuator port a3 to an actuator (not shown).
When the actuator port a3 pressure has reached a prescribed level, a hydraulic fluid pressure which is based on the diameter differential between a land b4 and a land b5 causes the spool B to be moved back towards the right. This shuts off communication between the pump port a2 and the actuator port a3, and the pressure in the actuator port a3 is maintained at the prescribed level.
In the solenoid proportional control valve according to this arrangement, the diameter of the orifice b2 is made very small to minimize the effect the flow of hydraulic fluid to the pilot chamber C has on the main flow path from the pump port a2 to the actuator port a3 and to eliminate pressure variations in the pilot chamber C arising from the pumping action of the hydraulic fluid from the pump port a2. In this type of solenoid proportional control valve the orifice b2 therefore quickly becomes blocked, making it impossible for the valve to perform its control function.
An idea of the present inventor was to provide a filter in the opening b3. However, when the opening b3 has a very small diameter, as shown in FIG. 10, filters quickly become clogged and need to be replaced at frequent intervals, making maintenance a problem. Although this could be prevented by making the opening b3 larger, as the opening b3 is situated in the outer wall of the pilot chamber C, increasing the diameter of the opening b3 would mean increasing the area of reduced wall thickness between the opening b3 and the horizontal opening b1, degrading the strength of the spool B.
FIGS. 11 and 12 illustrate another type of conventional spool control valve, which will now be described. Here, the valve is a solenoid proportional control valve 101, which FIG. 11 shows in cross-section, with an enlarged view of the main parts being shown by FIG. 12. With reference to FIG. 11, the solenoid proportional control valve 101 is provided with a valve housing 101, a valve housing 102, a proportional solenoid 103 provided as an integral part of the valve housing 102, and a spool 105 which slides reciprocally in a spool passage 104.
Formed inside the valve housing 102 are a high-pressure delivery port 106, a low-pressure drain port 107, a tank port 108 and a low-pressure drain port 109. Each of the ports 106 to 109 communicates with the spool passage 104. The coil 111 of the solenoid 103 is energized by control signals received via a cable 110, causing an armature 112 to move a prescribed distance to thereby move the spool 105 horizontally. Formed in the spool passage 104 from which they project inwardly are an annular delivery side sliding face 113, a tank side sliding face 114 and a drain side sliding face 115.
The spool 105 is moved by the armature 112 and is returned to its original position by the valve return spring 116. Formed on the peripheral surface of the spool 105 are first and second lands 117 and 118 which slide against the annular delivery side sliding face 113, a land 119 which slides against the tank side sliding face 114 and a land 120 which slides against the drain side sliding face 115.
An annular hydraulic pressure balance groove 121 is formed between the first and second lands 117 and 118. The hydraulic pressure in the hydraulic pressure balance groove 121 exerts a uniform peripheral pressure on the spool 105 to prevent axial deviation of the spool 105. The first and second lands 117 and 118 serve to define a delivery port 106 side high-pressure chamber 122 and a drain port 107 side low-pressure chamber 123, respectively.
As shown by the enlarged view in FIG. 12, a problem with this arrangement is that as a hydraulic pressure differential is used to produce a flow of hydraulic fluid from the high-pressure chamber 122 to the low-pressure chamber 123 (as indicated by the arrow R), particles of foreign matter CT such as dirt and the like get drawn in between the delivery side sliding face 113 and the delivery side second land 118. These particles build up over an extended period and cause the spool 105 to stick in the spool passage 104.
Japanese Unexamined Patent Publication Nos. 61(1986)-201903 and 63(1988)-195477 describe valve mechanisms designed to prevent the movement of the spool being stopped by such particles.
One such arrangement involves boosting the throughput of a proportional solenoid valve by equipping the valve with a stronger return spring 116 or other internal springs to enable the valve to remain functional even if a moderate amount of particles of foreign matter CT enters the valve. However, the drawback with such an arrangement is that it increases the overall size of the valve.